The present invention is directed to a method for the integrated series-interconnection of a plurality of thick-film solar cells located on an insulating transparent substrate.
Due to a relatively high current that is generated, of approximately 40 mA/cm.sub.2, solar cells constructed from crystalline silicon in the form of small surfaces of about 1 dm.sub.2 must be interconnected to one another, in order to avoid current losses. This is true of crystal wafers of single-crystal silicon whose sizes are limited by the method of growing the crystal.
It is possible to obtain larger surfaces of polycrystalline silicon. For example, these larger surfaces of polycrystalline silicon can be produced through a ribbon-drawing method. The larger surface polycrystalline silicon is sawn into wafers having a size of approximately 10 cm.times.10 cm, and subsequently soldered together, again in a serial fashion, with small metal bands. Although current losses can thereby be adequately avoided; the process, however, is involved and expensive.
A further drawback of crystalline silicon is that its low energy gap of E.sub.g =1.1 eV, is poorly suited for a solar spectrum having an intensity maximum at 1.45 eV. It would therefore be desirable if the low energy gap could be combined, in a tandem cell, with an energy gap lying sufficiently above 1.45 eV.
Such a possibility is established, for example, by the combination of polycrystalline silicon (E.sub.g 1.1 eV) with amorphous silicon (E.sub.g 1.75 eV). Silicon is a material that is neither affected by resource problems nor by environmental problems and therefore could be used as a semiconductor material in both sub-cells.
Referring to FIGS. 7 and 8, the structure and function of a tandem cell is illustrated. I denotes an upper solar cell of amorphous, hydrogenated silicon having an energy gap of E.sub.g1. II denotes a lower solar cell of polycrystalline silicon having an energy gap E.sub.g2 (E.sub.g1 greater than E.sub.g2). III denotes an optical coupler. A glass substrate 1 is indicated by reference numeral 1. The penetration depth of short-wave light is indicated by arrow 7. The penetration depth of long-wave light is indicated by arrow 8.
Amorphous silicon in the form of hydrogenated a-Si:H has an energy gap of, typically, 1.75 eV. The energy gap can be shifted somewhat (1.7 through 1.8 eV) by variations in the manufacturing conditions. As may be derived from a report by D. Morel et al in the Proceeding of the 18th Photovoltaic Specialists Conference (1985), pages 876 through 882, there are solar cells of amorphous silicon, even in an embodiment necessary for front cells comprising two transparent electrodes.
The silicon in an a-Si:H solar cell has a layer thickness of less than 0.5 .mu.m. The energy gap of the amorphous silicon in these cells can be somewhat increased by adding carbon. However, the layer thickness of the polycrystalline silicon needed for the lower sub-cell, by contrast, is 20 through 30 .mu.m (thick-film cell). The sunlight is then adequately absorbed.
Due to cost considerations, preferably, the technology that should be utilized to manufacture polycrystalline silicon thick films is technology that is analogous to thin-film technology. What is critical is that these sub-cells are manufactured so that they have an adequately smooth surface without requiring involved mechanical re-working. This allows them to be subsequently bonded to an a-Si:H cell through a simple laminating process. A significant advantage of thin-film solar cells is that by dividing them into narrow strips, they can be series-interconnected. This results in modules having a higher voltage and low current.
The series-interconnection of the thick-film cells in small surface units (strips cells) is also desirable in order to manage the low current losses in this part of the tandem solar cell. The series-interconnection in small surface units is likewise desirable in order to make the voltage of the modules variable based on external requirements. Heretofore, this has not been possible in an economic manner.
U.S. Pat. No. 4,745,078 discloses a method for thin-film solar cells wherein separating grooves are produced, for interconnection, in a thin-film applied to a substrate provided with front electrodes. A stripe pattern of, preferably, plastic is applied before the application of a metal electrode layer for providing series interconnections. Separating grooves are then produced, mechanically, next to the stripe pattern. The grooves are then filled with metal. The metal electrode layer is then deposited and, subsequently, the stripe pattern is again removed through a lift-off technique, whereby a metal electrode series interconnection structure is formed.
Another method for manufacturing series-interconnected modules of crystalline silicon, utilizing a mechanical separation, is proposed in German Patent Application No. P 37 27 825.8. In this method, narrow ridges that have already been provided on the substrate by selectively removing material from a glass substrate, are used for defining the stripe-shaped individual solar cells. The front electrodes are produced through a silk-screening method, and then the series-interconnection is formed via grooves in the rear contact.
Both of the methods, particularly the latter (due to the co-coating of the ridges), have the great disadvantage that they require involved etching and grinding processes. Moreover, a damage layer, that is high in defects, arises during the grinding process, and must be eliminated through an additional etching process. The methods, in addition to resulting in higher costs, have the drawback that due to the grinding process, edge eruptions in the layer are produced. Overall, such methods are not adapted to thin-film technology.